Power conditioning circuits are used to remove unwanted voltages and currents from alternating current voltage sources intended for supplying operating power to electronic equipment. Power conditioning sometimes refers to surge and spike protection. Another example of power conditioning pertains to removal of unwanted spectral components that may be of small amplitude with respect to the fundamental voltage of the power source. Specifically, power conditioning may refer to removal of spectral components either introduced by the power source or generated by the loads. Finally, power conditioning sometimes refers to regulation of the average amplitude of the power source voltage.
A series regulator is an apparatus that acts to control the voltage at some node in a network by adding a correction voltage to an existing voltage so that the sum of the voltages is controlled. A source of correction (or error correction) voltage is connected in series with an existing source of voltage. The correction voltage either acts to buck or to boost the existing voltage so that the discrepancy between the existing voltage and the desired voltage is removed. Voltage “bucking” is the addition of a opposite polarity voltage to an existing voltage, the result of which is a new voltage that has reduced magnitude with respect to the existing voltage. Voltage “boosting” is the addition of a same polarity voltage to an existing voltage, the result of which is a new voltage that has increased magnitude with respect to the existing voltage.
A feedback control system is a type of system that actively minimizes the error formulated as the difference between its desired behavior and its actual behavior. A feedback control system makes continuous comparison between the actual behavior of a system and a standard of desired behavior. From this comparison, a correction influence is derived that acts on the system to null the error.
An inverter is an apparatus that converts DC voltage into AC voltage. It may also be configured to first convert AC voltage into DC voltage, and second to convert DC voltage back into AC voltage. Inverters are frequently used in power conditioning and power control circuits to perform corrective operations on the incoming AC power.
A growing number of nonlinear loads in the electric utility power network has resulted in increasing waveform distortion of both voltages and currents in ac power distribution systems. Typical nonlinear loads are computer controlled data processing equipment, numerical controlled machines, variable speed motor drives, robotics, medical and communication equipment.
Utilities provide sinusoidal supply voltages. Nonlinear loads draw square wave or pulse-like discontinuous currents instead of the purely sinusoidal currents drawn by conventional linear loads. As a result, nonlinear current flows through the predominantly inductive source impedance of the electric supply network. Consequently, a non-linear load causes load current harmonics and reactive power to flow back into the power source. This results in unacceptable voltage harmonics and supply load interaction in the electric power distribution network.
The degree of current or voltage distortion can be expressed in terms of the magnitudes of harmonics in the waveforms relative to the fundamental magnitude. Total Harmonic Distortion (THD) is one of the accepted standards for measuring voltage or current quality in the electric power industry.
Aside from the waveform distortion, another imperfection common to utility electric power is the presence of spurious and random noise. Various electric appliances such as electric motors, radio and television receivers, and digital electronic computers and related appliances generate noise components that are non-harmonically related to the fundamental power line frequency. Finite isolation between the appliances and the public utility power grid results in spurious and random noise energy components traveling along the power grid. Further, electromagnetic radiation containing a wide variety of spectral components as well as broad spectrum noise couples into the power grid and propagates to the end user load equipment.
A wide variety of electrically powered appliances such as comprises home entertainment audio and video systems, test and measurement systems, hospital monitoring equipment, and computer systems have degraded operating performance when powered by AC power that contains typical levels of harmonic, spurious, and random noise components.
Another problem related to the nature of utility AC power pertains to the finite impedance of the AC power source as it appears at the load. When a number of separate pieces of electrical equipment are connected to the same AC power source there is a finite degree of coupling between the components at the point of common connection to the power source. The noise that each component generates and that leaks over to the AC input is allowed to enter each of the other components in the system. In this way each part of the system operates to degrade the performance of the remaining components. The AC power source offers only partial isolation between components because it has a finite source impedance at the point of common connection to the loads. An example of this is digital clock noise generated by a digital to analog converter in an audio playback system coupling through the power line connections and degrading the performance of a preamplifier in the same system.
Finite source impedance also inhibits the performance of electrical equipment by allowing the AC voltage sinewave to sag during high current transients. This results in higher levels of power supply ripple and compromised performance. An example can be seen in an audio amplifier application, where signal transients require high levels of current with sustained power supply voltage. In this case the finite power line impedance results in temporary collapsing of the AC sinewave voltage and a resulting distortion from the amplifier.
The unbalanced configuration of single phase AC power introduces additional problems for sensitive electronic equipment. There is significant radiation and coupling of AC power energy into downstream circuitry because the fields associated with unbalanced power are far reaching. In addition, the AC return current path utilizes the neutral line, which is eventually tied to earth ground. Since earth ground is the common node for each component in a system a certain degree of AC noise injection into the rest of the system is made possible by terminating the AC current path to this node.
A number of techniques have been used in an attempt to provide power line conditioning to address some of the foregoing problematic conditions. Passive filters, such as LC tuned filters, are often used because they are efficient and inexpensive. There are, however, a number of problems associated with passive filters. Practical constraints of size and cost limit passive filters to relatively simple topologies using relatively few components. The result is only a moderate suppression of AC power imperfections. In addition, the tuned resonators in passive filters have non-ideal behavior at frequencies other than the power line frequency. This manifests itself as undesirably high source impedance and results in poor current on demand characteristics and poor inter-component isolation.
Active power filters have been developed to resolve some of the problems associated with passive filters. Active power filters, or active power line conditioners (APLCs), inject signals into an ac system to cancel harmonics.
Active filters comprise one or two pulse width modulated inverters in a series, parallel, or series-parallel configuration (with respect to the load or supply). The inverters have a dc link, which can be a dc inductor (current link) or a dc capacitor (voltage link). It is necessary to keep the energy stored in the dc link (capacitor voltage or inductor current) at an essentially constant value. The voltage on the dc link capacitor can be regulated by injecting a small amount of real power into the dc link. The injected real power compensates for the switching and conduction losses inside the APLC.
One problem associated with active filters is that it is expensive and complicated to generate a reference signal for the feedback control that is sufficiently monochromatic and free of noise, and also phase locked to the incoming AC power. Customarily the digital clocks and phase detectors used to synchronize a free running oscillator to the AC line voltage generate considerable noise. The low loop bandwidth required for operation at power line frequencies results in inordinately long settling times and poor transient response.
Another problem with active filters is that of stability. The feedback control loop acting to monitor and correct error in the output signal with respect to the reference is typically band limited to prevent oscillation of the loop. The spectral purity and desirably low output impedance of the output power is limited to the bandwidth of the loop.
Alternatively, the AC power can be completely regenerated from a DC power supply operating from the AC power line. In this approach a pure sinewave source is amplified to the line voltage level by class A-B amplifiers. This approach suffers from very low efficiency, theoretically not to exceed 50%, and usually much lower values are realized due to practical circumstances. In addition, the source oscillator is typically derived from a digitally sampled sinewave and is corrupted by quantization impurities when reconstructed.